This is a U.S. national stage of application No. PCT/FR01/00203, filed on Jan. 22, 2001. Priority is claimed on that application and on the following application: Country: France, Application No.: 00/00865, Filed: Jan. 24, 2000.
The present invention relates to the known field of bottom-to-surface links of the type comprising a vertical underwater pipe referred to as a xe2x80x9criserxe2x80x9d connecting the bottom of the sea to a floating support installed on the surface.
Once the depth of water becomes large, production fields and in particular oil fields are generally worked from a floating support. In general a floating support has anchor means to enable it to remain in position in spite of the effects of currents, winds, and swell. It generally also has means for storing and processing oil and means for unloading to off-loading tankers. These supports are known as floating production storage off-loading (FPSO) supports and numerous variants have been developed such as submersible pipe alignment rigs (SPARs), which are long floating cigar-shaped objects held in position by catenary anchoring, or tension leg platforms (TLPs) where said legs are generally vertical.
Wellheads are often distributed over an entire field and production pipes together with water injection lines and control cables are disposed on the sea bed heading towards a fixed location with the floating support being positioned on the surface vertically thereabove.
Some wells are situated vertically below the floating support and the inside of the well is then directly accessible from the surface. Under such circumstances, the wellhead fitted with its xe2x80x9cChristmas treexe2x80x9d can then be installed on the surface on board the floating support. It is then possible using a derrick installed on said floating support to perform all of the drilling, production, and maintenance operations required by the well throughout its lifetime. This is said to be a xe2x80x9cdryxe2x80x9d wellhead.
To maintain the riser fitted with the dry wellhead in a substantially vertical position, it is appropriate to exert upward traction which can be applied either by a cable tensioning system using hydraulic actuators or winches installed on the floating support, or else by using floats that are distributed at various depths along the riser, or indeed to use a combination of both techniques.
U.S. Pat. No. 2,754,011 in the name of IFP describes a barge and a guide system for a riser, which riser is fitted with floats.
In some oil wells, the depth of water exceeds 1500 meters (m) and can be as much as 3000 m, so the weight of a riser over such a depth requires forces that can reach or even exceed several hundreds of (metric) tonnes in order to keep them in position. Use is made of buoyancy elements of the xe2x80x9ccanxe2x80x9d type which are added to underwater structures, mainly to risers connecting the surface to very great depths (1000 m to 3000 m). The underwater pipe then consists in a rising column comprising an underwater pipe assembled with at least one float comprising a can surrounding said pipe coaxially and having said pipe passing through it.
The means joining said can to said pipe preferably include a leakproof hinge assembled around said pipe at at least one of the top and bottom orifices of said can.
The floats in question are of large dimensions, and in particular of a diameter in excess of 5 m, and a length in the range 10 m to 20 m, and their buoyancy can be as much as 100 tonnes.
They are generally placed in a string one beneath another.
In general, the floats extend over a length corresponding to no more than about 10% of the length of the bottom-to-surface link, and in particular over a length of 100 m to 200 m.
The riser is put under tension by the floats and is guided, preferably at the floating support, by roller guides situated in a plane to hold and guide the riser relative to the floating support. Cable tensioning means acting as guides can be used.
FR 2 754 021 discloses a device for guiding a riser which is fitted at its top with floats, the device comprise wheels enabling the riser to slide vertically, and also enabling it to pivot about a horizontal axis, while guiding its horizontal displacements, such that the movements of the riser in horizontal translation follow substantially those of the floating support. Thus, FR 99/10417 discloses an improved guide device having wheels and friction pads disposed radially around the pipe. Finally, various guide systems are known that require tensioning to be performed by cable.
The entire riser then behaves like the string of an instrument under tension between the bottom of the sea and the point situated on the axis of the guidance system level with the floating support.
The riser is subjected to the effects of swell, of current, and in addition to the horizontal movements of said floating support which is itself likewise subjected to the same effects. Water movements in the depth of water create drag effects on the riser structure and on its floats, thereby giving rise to large forces of variable direction.
In certain configurations of water particle movement, interaction effects occur between the fluid and the riser which give rise to vortices being shed alternately from opposite sides of the riser as shown in FIG. 13.
When the period of these alternating vortex separations is close to the natural excitation periods of the riser, a phenomenon known as xe2x80x9ccapturexe2x80x9d occurs which causes the riser to vibrate.
The intensity of the vibrations generated by shedding vortices during xe2x80x9ccapturexe2x80x9d increases with increasing length of riser from which vortices are being shed simultaneously on the same side. This length is referred to by the person skilled in the art as the xe2x80x9ccorrelation lengthxe2x80x9d.
If the riser is considered as being an instrument string that is attached at both ends, its overall behavior is subject to transverse displacements of several meters presenting a natural excitation frequency and harmonics corresponding to a mode referred to as xe2x80x9cguitar modexe2x80x9d, i.e. the riser vibrates between its two ends like a guitar string.
For risers without floats whose section characteristics (in particular diameter, second moment of area, stiffness) are substantially uniform or continuous, the only vibration observed is vibration of this xe2x80x9cguitarxe2x80x9d type as shown in FIGS. 3 and 4.
Similarly, for a high-buoyancy riser as in WO 99/05389 in which buoyancy is distributed over the entire length of the riser to obtain buoyancy of about 95% to 98%, the only vibrations observed are in xe2x80x9cguitarxe2x80x9d mode.
In WO 95/27101 and WO 99/05389, stabilizers are described for risers that are subjected to xe2x80x9cguitarxe2x80x9d type vibrations associated with the disturbing effects of vortices or turbulence around the riser.
In WO 99/05389, the floats are distributed along the entire length of the riser in the form of a cylindrical shell of syntactic foam. The stabilizer consists in modifying the shape of the floats in the top portion corresponding to the depth of water which is subject to swell, so as to obtain a non-cylindrical surface of hexagonal section. The solution proposed in WO 99/05389 reduces the volume of the floats in the top portion and thus reduces their buoyancy compared with the remainder of the riser for equivalent overall size. That type of plane geometry modification has the effect of increasing drag and decreasing the excitation induced by shedding vortices, and stabilizes the riser solely by absorbing energy.
In WO 95/27101, a tubular pipe that is likewise subjected to xe2x80x9cguitarxe2x80x9d mode vibration only, is stabilized by being fitted with a plurality of perforated envelopes at different levels around the standard portion of the pipe. These perforated envelopes are slidable around the pipe so as to be placed specifically at the locations of zones that are subjected to vibrations which correspond to vibration antinodes.
In U.S. Pat. No. 4,768,984, a stabilizer is described for a riser that is provided with a free buoy at its head, said buoy supporting an installation that includes a working platform. The surface installation creates unbalance by raising the center of gravity relative to the center of thrust or of buoyancy. Attempts are therefore made to bring the center of gravity and the center of buoyancy of the top portion of the pipe back into proximity and preferably into coincidence by adding a trellis or reinforcing structure constituting a mass below the float in order to compensate the weight of the structure above the float. The trellis or reinforcing structure has a special shape to avoid giving rise to any excessive increase in disturbances in the zone in question.
The present invention relates to stabilizing a bottom-to-surface link of a type that is different from those described in the prior art, and it proposes a novel solution for stabilization.
The present invention relates to a bottom-to-surface link comprising at least one underwater pipe having at least one float, and preferably a plurality of floats in a string, said floats being constituted by cans surrounding said pipe, and located in the top underwater portion of the pipe, said pipe being held at the surface by a guide device, preferably on a floating support, and the portion situated beneath the float zone being devoid of floats for the most part or completely.
As mentioned above, the term xe2x80x9cguide devicexe2x80x9d is used herein to designate devices known to the person skilled in the art for allowing the riser to slide vertically, and also for allowing it to pivot about a horizontal axis while guiding its horizontal displacements, so that they are controlled and preferably substantially follow those of the floating support, where appropriate.
The inventors have discovered that with this type of riser having floats near the top only and having a support and guide system at the surface, a vibration phenomenon can be observed that is very different and that is accentuated, in which the top portion corresponding substantially to the length of the floats behaves like a pendulum whereas the bottom portion beneath the floats behaves in a manner that approximates to a xe2x80x9cguitarxe2x80x9d type phenomenon. The xe2x80x9cpendulumxe2x80x9d type behavior of the top portion of the riser is favored by the difference between the second moments of area between the two sections of the riser. This xe2x80x9cpendulumxe2x80x9d movement has a very significant influence on the behavior of the riser as a whole and coupling is then observed between xe2x80x9cpendulumxe2x80x9d mode and xe2x80x9cguitarxe2x80x9d modes, as shown in FIGS. 6 and 7. The overall behavior of the riser is then particularly sensitive to any excitation which tends to generate swinging motion in the top portion of the riser.
Thus, the problem of the invention is to prevent or reduce the appearance of vibration corresponding to combined xe2x80x9cguitar-and-pendulumxe2x80x9d type modes on risers that are tensioned by floats situated near the top, whenever they are excited by swell and current or indeed by the horizontal displacements of the barge, by reducing or preferably eliminating the pendulum-like behavior of the top portion of the riser.
More particularly, the inventors have shown that the problem posed is to avoid or reduce hydrodynamic phenomena that give rise to excitation of vibratory modes in the coupled xe2x80x9criser-and-floatxe2x80x9d system, and to avoid or reduce the response of the coupled xe2x80x9criser-and-floatxe2x80x9d system to the excitations.
Since this is typically a problem of the xe2x80x9cfluid-and structurexe2x80x9d coupling type, the interaction between the two aspects, i.e. excitation and system response, is large. The hydrodynamic phenomena and the movements of the barge act on the structure which in turn reacts on the excitation from which the hydrodynamic phenomena originate. The invention thus relates to installing devices that provide a solution to one or the other or both aspects of the problem.
The problem of the present invention thus relates, in a first aspect, to eliminating or reducing the excitation of vibratory modes of the coupled xe2x80x9criser-and-floatxe2x80x9d system. The inventors have shown that in this first aspect, the problem relates solely to using devices that have an influence on excitation of hydrodynamic origin of the vibratory modes due to vortices being shed from the outlines of the floats or the riser.
In a second aspect, the problem of the present invention also relates to eliminating or reducing the response of the coupled xe2x80x9criser-and-floatxe2x80x9d system to the excitations due to the hydrodynamic phenomena or to the horizontal movements of the barge.
The inventors have shown that in this second aspect, the problem relates to implementing devices seeking to modify the frequencies of the vibration modes of the coupled xe2x80x9criser-and-floatxe2x80x9d system so as to move away from the frequencies at which vortices are shed, thereby avoiding the xe2x80x9ccapturexe2x80x9d phenomenon.
To do this, the present invention provides a bottom-to-surface link comprising at least one underwater pipe having at least one float and preferably a plurality of floats in a string, said floats being constituted by cans surrounding said pipe and being located in the top underwater portion of the pipe, said pipe being held at the surface by a guide device, preferably on a floating support. The portion situated beneath the float zone is thus devoid of floats for the most part, or even completely.
According to the present invention, said link includes at least one stabilizer situated in the top portion of the pipe and constituted by:
the bottom portion of the float zone, preferably on or level with the lowest float; and
the transition zone between the floats and the standard portion of substantially constant diameter of said pipe.
Said transition zone between the riser and the float(s) corresponds to a zone where the mechanical characteristics of the pipe (diameter, section or second moment of area of the pipe) decrease progressively going downwards until the standard part of the pipe is reached, corresponding to the part of the pipe that is of substantially constant diameter and situated beneath said transition zone.
The invention takes advantage of the very particular pendulum-like behavior of this type of bottom-to-surface link to contribute to stabilizing the system.
Stabilizers of the invention are advantageously located at the bottom end of the float zone, or beneath the string of floats, since the inventors have shown that this location corresponds to the zone where excitation from vortex shedding predominates. This excitation is closely tied to the amplitude of the horizontal displacement of the riser; and in this location the lever arm relative to the axis of rotation of the pendulum-type motion is greater. This location thus increases the stabilizing effect of the stabilizer. In addition, said location constitutes a singular point of said pipe since tension is at a maximum there.
Stabilizers of the invention thus make it possible to reduce or even eliminate both the excitation and the response of the vibratory modes of the coupled xe2x80x9criser-and-floatxe2x80x9d system.
More particularly, said stabilizer comprises at least one device selected from:
an energy absorber device;
a device increasing the mass of water entrained during movement thereof; and
a device lowering the center of gravity of said top portion of the pipe.
Preferably, the stabilizer combines at least two and preferably three of the effects selected from:
energy absorption;
increasing the mass of water entrained during movements thereof; and
lowering the center of gravity of said top portion of the pipe.
In an embodiment, energy is absorbed by a structural element that increases the contact area with the water and/or that creates a contact area with the water that is not cylindrical about the axis of said pipe.
Thus, the structural element can be constituted by:
a modification of the shape of the surface of the float or the pipe, so that its shape is not cylindrical about the axis of said pipe; or
an additional structural element associated with the surface of the float or of the pipe.
Said additional structural element increasing the contact area of the float or of the pipe with the water can present a surface having a shape that is three-dimensional or plane. This surface having a three-dimensional shape is preferably a surface that is not parallel to the cylindrical surface of the pipe.
Advantageously, said stabilizer comprises one of the following embodiments, taken separately or in combination:
the non-cylindrical shape of the outside surface of a float or a portion of the pipe; and
a helical ramp surrounding said float or said pipe.
As an example of said device lowering the center of gravity of the top portion of the pipe, mention can be made of a stabilizer having an additional mass situated in or around a float or surrounding said pipe.
In an embodiment, said stabilizer comprises a caisson, that preferably surrounds said pipe coaxially and whose outside surface has perforations. The term xe2x80x9cperforationxe2x80x9d is used to cover any gap or opening that allows water to pass into the caisson.
In certain embodiments, particularly when the stabilizer is a perforated caisson, it combines various aspects, specifically it consists in a device that lowers the center of gravity since it is situated in the bottom portion of the float zone, and in a device that increases the mass of water that is entrained during movement since it contains water, and in a device that absorbs energy by increasing drag due to the perforations in its surface.
As an example of a device providing additional mass, mention can be made of a non-perforated float that is partially or completely filled with water.
The invention thus consists in using in particular either additional devices such as perforated caissons or helixes, or else non-cylindrical floats of various shapes, or indeed a combination of these two types of solution.
In an embodiment, the float zone comprises a plurality of floats, and preferably at least four.
The stabilizer is preferably situated on or level with the lowest float, or in the transition zone immediately below the lowest float.
Said structural elements of three-dimensional shape absorb energy so as to limit the appearance of vortex shedding and/or reduce correlation lengths, while increasing the mass of water that is entrained.
Stabilizers operating by absorbing energy and increasing entrained water mass also contribute advantageously to modifying, and preferably reducing, the natural vibration frequencies of the xe2x80x9criser-and-floatxe2x80x9d system, and thus to reducing the response of the system to excitation due to hydrodynamic phenomena or to horizontal movements of the barge.
The invention also comprises implementing a stabilizer enabling the inertia of the string of floats to be increased by lowering its center of gravity, and also in a device enabling the mass of water that is entrained during its movement to be increased, which mass of water is known to the person skilled in the art as xe2x80x9cadded massxe2x80x9d. These two types of stabilizer can coincide in a single device as mentioned above.
These devices for lowering the center of gravity and increasing the xe2x80x9cadded massxe2x80x9d have not only a favorable effect on the vibration frequencies of the coupled xe2x80x9criser-and-floatxe2x80x9d system, but also enable the top portion of the riser to be stabilized by increasing its inertia, thereby contributing to damping its swinging motion. These devices are also preferably installed on the bottom portion of the string of floats or beneath the floats where they are particularly effective since the dynamics of the coupled system are governed mainly by the swinging motion of the floats, and thus has a large influence on the natural frequency of oscillation of the moving riser, considered as a whole.
For risers that are grouped together in a bundle, vibratory modes are no longer excited solely by vortices being shed from the riser in question, but also due to flow interactions with other risers in the bundle. Implementing stabilizers of the present invention thus makes it possible to disorganize the wakes around the various risers and advantageously contributes to reducing the excitation of vibratory modes in adjacent risers.
The present invention also provides a bottom-to-surface link comprising a plurality of underwater pipes of the invention, i.e. comprising at least one stabilizer and grouped together in a bundle.